This invention relates in general to a mass transfer or distillation column and, more particularly, to a downcomer within the column for feeding liquid from one tray to an underlying tray.
Conventional downcomers are used in mass transfer or distillation columns to provide a pathway for liquid to descend from one tray to an underlying tray. The trays used in conjunction with the downcomers are horizontally disposed plates that are perforated so that they are permeable to ascending vapor. The vapor passes upwardly through the perforations in the trays and interacts with liquid flowing across the top surface of the trays. Sieve trays and valve trays are examples of commonly employed trays used in conjunction with downcomers.
In single pass arrangements, the downcomers are positioned on opposite ends of adjacent trays so that the liquid flows completely across one tray before it enters the associated downcomer and is fed onto the underlying tray. The liquid then flows in the opposite direction across the underlying tray and enters the associated downcomer for passage to the successive underlying trays in a back and forth manner. In multiple pass arrangements, more than one downcomer is placed at preselected positions on at least every other tray and the liquid is divided into separate streams for passage through the respective downcomers.
The downcomers used in mass transfer columns typically have a plurality of generally vertical, but sometimes sloped, walls which are joined together to form an open-ended enclosure. The downcomer walls are fabricated from solid sheet-like material such as metal and one or more of the walls may be formed by the column shell. The downcomer walls thus channel the liquid entering the downcomer downwardly for discharge through the bottom of the downcomer onto the underlying tray. At least one of the downcomer walls is joined to the edge of the associated tray and a liquid accumulation weir is usually formed by an upward extension of the downcomer wall.
The bottom or downstream end of conventional downcomers is at least partially open to form a liquid discharge outlet which is spaced a preselected distance above the underlying tray. Typically, the downcomer outlet is positioned below the level of the liquid that accumulates on the underlying tray to create a seal against vapor entering the downcomer outlet.
Because the downcomer outlet must typically be located below the liquid level on the underlying tray, the area available to accommodate the discharge of liquid from the downcomer is necessarily restricted. In most instances, all of the liquid which is discharged from the downcomer outlet must pass through the vertical area bounded at the top by the bottom edge of the forward downcomer wall, at the bottom by the tray upper surface and at the sides by the column shell. If the vertical area is too small in relation to the volumetric flow rate of liquid entering the downcomer, liquid will back up in the downcomer and flooding of the downcomer may result. This restricted outlet flow area can thus undesirably limit the liquid handling capacity of the downcomer.
Increasing the restricted outlet flow area by shortening the downcomer is a generally unsatisfactory approach to increasing liquid handling capacity because the shortened downcomer provides less opportunity for entrained vapor to disengage from the liquid prior to being discharged from the downcomer. In addition, if the downcomer outlet is raised above the liquid level on the underlying tray, the vapor would be permitted to enter the downcomer outlet and disrupt the liquid flow dynamics. Although the vapor seal can be maintained by increasing the height of the weir and thus raising the liquid level on the tray, less open space remains above the liquid froth level on the tray for disengagement of the entrained liquid from the ascending vapor. As a result, the operational efficiency of the downcomer and tray structure is reduced as the entrained liquid is recycled from one tray to an overlying tray.
Another approach to increasing the liquid handling capacity of downcomers involves the use of a sump in the tray area below the discharge outlet of the downcomer. The sump increases the distance of separation between the tray and the downcomer outlet, thereby increasing the area available for liquid flow. While the tray sump can reduce the pressure drop and increase the liquid handling capacity of the downcomer, it prevents the use of raised seal pans or tray decks which are designed to increase the vapor handling capacity of the tray. Another disadvantage to the use of these tray sumps is the added complexities in design and installation of these internals.
A need has thus arisen for a downcomer which has an increased liquid handling capacity but which does not suffer the disadvantages associated with conventional downcomers.